1. Field of the Invention
The present invention relates generally to a mobile terminal, and in particular, to an apparatus and method for generating transmission and reception local oscillation signals.
2. Description of the Related Art
In a mobile terminal, a local oscillation signal for transmission (TX local oscillation signal) is used to upconvert the frequency of a transmission signal, whereas a local oscillation signal for reception (RX local oscillation signal) is used to downconvert the frequency of a received signal. The TX and RX local oscillation signals are generated by a PLL (Phase Locked Loop) block and provided to a radio transmitter and a radio receiver, respectively.
In a conventional TDD (Time Division Duplex) mobile terminal, a TX local oscillation signal and an RX local oscillation signal are selectively provided by a PLL block 8 as shown in FIG. 1. The PLL block 8 synchronizes the frequency and phase of its output signal to those of a reference signal or an input signal.
Referring to FIG. 1, the PLL block 8 is comprised of a frequency synthesizer 2, a low pass filter (LPF) 4, and a voltage controlled oscillator (VCO) 6. A controller (not shown) feeds an enable signal EN, data DT, a clock signal CLK to the frequency synthesizer 2 of the PLL block 8, so that the PLL block 8 may selectively provide: a TX local oscillation signal (TX LO) to a radio transmitter in a transmission mode, and an RX local oscillation signal (RX LO), to a radio receiver in a reception mode. The data DT controls values for the TX and RX local oscillation signals. The frequency synthesizer 2 receives the data DT, in an active logic low state of the enable signal EN, in response to the clock signal CLK. The frequency synthesizer 2 generates a signal with a very stable frequency, by synthesizing a signal of a reference signal source received from a reference signal generator, based on the data DT. The LPF 4 filters the signal received from the frequency synthesizer 2 and provides a voltage value corresponding to the filtered signal to the VCO 6. The VCO 6 generates a signal with an oscillation frequency corresponding to the voltage value, that is, TX LO or RX LO. TX LO and RX LO are provided to the radio transmitter and the radio receiver, respectively. Then the TX and RX oscillation signals are fed back to the frequency synthesizer 2 at the same time.
It is possible to selectively generate a TX local oscillation signal and an RX local oscillation signal using the single PLL block 8 in the conventional TDD mobile terminal for the following reason. The TDD mobile terminal has a frame structure as shown in FIG. 2. Referring to FIG. 2, the frame is a GSM (Global System for Mobile telecommunication) TDMA (Time Division Multiple Access) frame. Each GSM TDMA frame is 4.615 ms in duration and has a gap of about 1.154 ms between a TX burst period and an RX burst period. The 1.154 ms gap is sufficient for the PLL block 8 to secure a time required to switch frequencies from TX LO, and RX LO, local oscillation signals (hereinafter, referred to as “PLL lock-up time” or “frequency switching time”). Therefore, the TDD mobile terminal can generate TX LO in a transmission mode and RX LO in a reception mode by use of the single PLL block 8.
However, a propagation delay that occurs in a multi-time slot mode, between a mobile terminal, or between a mobile terminal and a base station must be considered when multi-slot standards are supported as in HSCSD (High-Speed Circuit Switched Data) and GRS (General Radio Service), or a satellite-based TDD scheme is employed.
The conventional mobile communication system does not need a short PLL lock-up time because it mainly provides voice transmission. On the contrary, future mobile communication systems will be configured to additionally provide transmission of pictures and data communication. For the resulting use of multi-time slots in a TDMA frame, the PLL lock-up time should be reduced. However, the conventional method shown in FIG. 1 has limitations in reducing the PLL lock-up time because as the PLL lock-up time decreases, the phase noise performance of the PLL block deteriorates.
Therefore, there exists a need for an apparatus and a method that provides a shorter frequency switching time, or a shorter PLL lock-up time, which allows the PLL block to provide a transmission of pictures and data communication.